Noise reduction systems similar to the type described in U.S. Pat. No. 3,789,143, issued to David E. Blackmer on Jan. 29, 1974, generally dynamically compress (encode) the signal prior to recording or trasmission, and dynamically expand the signal after transmission or upon playback. This type of noise reduction system is generally known as a "companding" (compress/expand) system. Systems which use the companding process do not remove noise present in the original signal, they only prevent additional noise from being added by the recording or transmission process. Assuming perfect processing, the processed output signal is identical to the original signal in signal content and unwanted noise.
Single-ended devices such as dynamic range expanders are sometimes used to reduce noise. Expanders similar to the type described in U.S. Pat. No. 4,177,356, issued to C. Rene Jaeger and Lawrence E. Blakely on Dec. 4, 1979, increase the level of loud musical passages (upward or positive expansion) and decrease the level of soft musical passages (downward or negative expansion). During downward expansion, the noise floor of the expanded output signal is reduced to a level below that of the original signal by an amount determined by the expansion ratio. The reverse is also true. During positive expansion, the noise floor of the expanded output signal may increase to a level above the noise floor level in the original signal or source of the original signal. The larger the expansion ratio, the more the noise floor will be reduced during downward expansion. Large expansion ratios may result in undesirable noise modulation of the noise floor and any other unwanted noise that may coexist with the original signal under full expansion processing. The expansion ratio is, therefore, kept typically small allowing the noise floor of the expanded output signal to be only slightly reduced from the noise floor of the original signal when the original signal is either low in amplitude (soft) or nonexistent. The use of dynamic range expanders to reduce noise primarily affects only the noise floor of the input signal and cannot remove other unwanted noise present in the input signal. In addition, the original signal will be expanded. This increase in dynamic range in many applications may be undesirable. In broadcasting, for instance, most material will be compressed without being part of a companding process to limit the dynamic range before transmission. Limiting the dynamic range in broadcasting makes the station appear louder to its listening audience by providing a stonger, easier-to-find signal. Compression will increase the level of the noise floor from the original signal or source of the original signal to a more noticeable level. In addition, other unwanted noise present in the original signal will be more pronounced.
Another type of single-ended noise reduction system is a dynamically variable (program dependent) low pass filter. This type of noise reduction system can actually reduce the level of noise unavoidably present in the program material by dynamically restricting the system bandwidth to the bandwidth required to pass the program material. At very low input signal levels or with no input signal, the bandwidth may be reduced to 1 KHz or less. Devices of this type are typically limited in the amount of signal-to-noise (S/N) improvement that can be realized. They do provide dynamic range integrity between the processed output and the unprocessed input but offer little better than 14 db of S/N improvement.